Abrasive-impregnated cutting structure having anisotropic wear resistance and drag bit including same

ABSTRACT

An abrasive-impregnated cutting structure for use in drilling a subterranean formation is disclosed. The abrasive-impregnated cutting structure may comprise a plurality of abrasive particles dispersed within a substantially continuous matrix, wherein the abrasive-impregnated cutting structure exhibits an anisotropic wear resistance. One or more of the amount, average size, composition, properties, shape, quality, strength, and concentration of the abrasive particles may vary within the abrasive-impregnated cutting structure. Anisotropic wear resistance may relate to a selected direction, such as, for example, one or more of an expected direction of engagement of the abrasive-impregnated cutting structure with the subterranean formation and an anticipated wear direction. Anisotropic wear resistance of an abrasive-impregnated cutting structure may be configured for forming or retaining a formation-engaging leading edge thereof. A rotary drag bit including at least one abrasive-impregnated cutting structure is disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 12/396,556,filed Mar. 3, 2009, now U.S. Pat. No. 8,333,814, issued Dec. 18, 2012,which is a continuation of application Ser. No. 11/044,782, filed Jan.27, 2005, now U.S. Pat. No. 7,497,280, issued Mar. 3, 2009, thedisclosure of each of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fixed cutter or drag type bits fordrilling subterranean formations. More specifically, the presentinvention relates to impregnated drag bits for drilling rock formations.

2. State of the Art

So-called “impregnated” drag bits have been used conventionally fordrilling rock formations that are hard, abrasive, or both. Moreparticularly, conventional earth boring drag bits withdiamond-impregnated cutting structures, commonly termed “segments,” or,alternatively, discrete diamond-impregnated cutting structures have beenemployed to bore through very hard and abrasive formations, such asbasalt, dolomite, and hard sandstone. These conventional impregnateddrag bits typically employ a cutting face comprising adiamond-impregnated material, which refers to an abrasive particle ormaterial, such as natural or synthetic diamond grit, uniformly dispersedwithin a matrix of surrounding material. As a conventional impregnateddrag bit drills, the matrix wears to expose the abrasive particles, theabrasive particles also wear, and worn abrasive particles may be lostand new abrasive particles, which were previously surrounded by matrixmaterial, may be exposed.

In fact, many conventional diamond impregnated segments may be designedto release, or “shed,” such diamonds or grit in a controlled mannerduring use of the drag bit. As a layer of diamonds or grit is shed fromthe face, underlying diamonds are exposed as abrasive cuttings and thediamonds that have been shed from the drag bit wear away the exposedcontinuous phase of the segment in which the interior diamonds aresubstantially uniformly dispersed until the entire diamond-impregnatedportion of the bit has been consumed. Thus, drag bits withdiamond-impregnated segments may maintain a substantially constantboring rate or rate of penetration, assuming a homogeneous formation, aslong as diamonds remain exposed on such segments.

Regarding conventional abrasive-impregnated cutting structures, theabrasive material with which the continuous matrix material isimpregnated preferably comprises a hard, abrasive and abrasion-resistantparticulate material, and most preferably a super-abrasive material,such as natural diamond, synthetic diamond, or cubic boron nitride.

The impregnated segment may include more than one type of abrasivematerial, as well as one or more sizes or quality grades of abrasivematerial particles. In conventional abrasive-impregnated cuttingstructures, the abrasive is substantially homogeneously distributed(i.e., not segregated) within the continuous matrix material. Thecontinuous matrix material may be chosen for wettability to the abrasiveparticles, mechanical properties, such as abrasion resistance, or both,and may comprise one or more of copper, a copper-based alloy, nickel, anickel-based alloy, cobalt, a cobalt-based alloy, iron, an iron-basedalloy, silver, or a silver-based alloy.

Two general approaches are conventionally employed to fabricate dragbits having abrasive-impregnated cutting structures.

In a first approach, an abrasive-impregnated cutting structure may becast integrally with the body of a drag bit, as by low-pressureinfiltration. For instance, one conventional abrasive-impregnatedcutting structure configuration includes placing abrasive material intoa mold (usually mixed with a molten wax) as by hand-packing, as known inthe art. Subsequently, the mold may be filled with other powders and asteel core and the entire assembly heated sufficiently to allow for ahardenable infiltrant, such as a molten alloy of copper or tin toinfiltrate the powders and abrasive material. The result, upon theinfiltrant cooling and hardening, is a bit body, which hasabrasive-impregnated cutting structures bonded thereto by the continuousmatrix of the infiltrant.

In a second approach, the abrasive-impregnated cutting structures may bepreformed or fabricated separately, as in hot isostatic pressureinfiltration, and then brazed or welded to the body of a drag bit. Thus,conventional abrasive-impregnated cutting structures may be formed asso-called “segments” by hot-pressing, infiltration, or the like, whichmay be brazed or otherwise held into a bit body after the bit body isfabricated. Such a configuration allows for the bit body to includeinfiltrants with higher melting temperatures and to avoid damage to theabrasive material within the abrasive-impregnated cutting structuresthat would occur if subjected to the higher temperatures.

As known in the art, diamond impregnated segments of drag bits may betypically secured to the boring end, which is typically termed the“face,” of the bit body of the drag bit, oriented in a generally radialfashion. Impregnated segments may also be disposed concentrically orspirally over the face of the drag bit. As the drag bit gradually grindsthrough a very hard and abrasive formation, the outermost layer of theimpregnated segments containing abrasive particles wear and mayfracture, as described above. For instance, U.S. Pat. No. 4,234,048 (the“'048 patent”), which issued to David S. Rowley on Nov. 18, 1980,discloses an exemplary drag bit that bears diamond-impregnated segmentson the crown thereof. Typically, the impregnated segments of such dragbits are C-shaped or hemispherically shaped, somewhat flat, and arrangedsomewhat radially around the crown of the drag bit. Each impregnatedsegment typically extends from the inner cone of the drag bit, radiallyoutwardly therefrom and up the bit face to the gage. The impregnatedsegments may be attached directly to the drag bit during infiltration,or partially disposed within a slot or channel formed into the crown andsecured to the drag bit by brazing.

Alternatively, conventional discrete, post-like cutting structures aredisclosed in U.S. Pat. Nos. 6,458,471 and 6,510,906, both of which areassigned to the assignee of the present invention and each of thedisclosures of which are incorporated, in their entirety, by referenceherein.

U.S. Pat. No. 3,106,973 issued to Christensen on Oct. 15, 1963,discloses a drag bit provided with circumferentially and radiallygrooves having cutter blades secured therein. The cutter blades havediamond impregnated sections formed of a matrix of preselectedmaterials.

U.S. Pat. No. 4,128,136 issued to Generoux on Dec. 5, 1978, discloses adiamond coring bit having an annular crown and inner and outerconcentric side surfaces. The inner concentric side surface of the crowndefines a hollow core in the annular crown of the bit for accommodatinga core sample of a subterranean formation. The annular crown is formedfrom a plurality of radially oriented composite segments impregnatedwith diamonds radially and circumferentially spaced apart from eachother by less abrasive spacer materials.

U.S. Pat. No. 6,095,265 to Alsup discloses an adaptive matrix includingtwo or more different abrasive compositions in alternating ribs or instaggered alternating zones of each rib to establish different diamondexposure in specified areas of the bit face. Alsup further disclosesthat the abrasive compositions for adaptive matrix bits contain diamondand/or other super-hard materials within a supporting material. Thesupporting material may include a particulate phase of tungsten carbideand/or other hard compounds, and a metallic binder phase of copper orother primarily non-ferrous alloys. Alsup discloses that the propertiesof the resulting metal-matrix composite material depend on both thepercentage of each component and the processing that combines thecomponents. Further, Alsup discloses that the size and type of thediamonds, carbide particles, binder alloy or other components can alsobe used to effect changes in the overall abrasive or erosive wearproperties of the abrasive composition. Additionally, such adjacent“hard” and “soft” ribs may purportedly facilitate fluid cleaning in andaround the ribs.

U.S. Pat. No. 6,458,471 to Lovato et al., assigned to the assignee ofthe present invention and the disclosure of which is incorporated hereinin its entirety by reference thereto, discloses cutting elementsincluding an abrasive-impregnated cutting structure having an associatedsupport member, wherein the support member is securable to an earthboring rotary-type drag bit body and provides mechanical support to thecutting structure.

U.S. Pat. No. 6,742,611 to Illerhaus et al., assigned to the assignee ofthe present invention and the disclosure of which is incorporated hereinin its entirety by reference thereto, discloses a first cutting elementsegment formed of a continuous-phase solid matrix material impregnatedwith at least one particulate superabrasive material, the first cuttingelement segment juxtapositioned with at least one second cutting elementsegment formed of a continuous-phase solid matrix material to form alaminated cutting element. Preferably, the at least one second cuttingelement segment is essentially devoid of impregnated superabrasive orabrasive particles. Alternatively, the at least one second cuttingelement segment can be impregnated with a preselected, secondary,particulate superabrasive material that results in the at least onesecond segment being less abrasive and less wear resistant than the atleast one first abrasive segment.

While the above-discussed conventional abrasive-impregnated cuttingstructures and drag bits may perform as intended, it may be appreciatedthat improved abrasive-impregnated cutting structures and drag bitswould be desirable. Further, it would be desirable to improveabrasive-impregnated cutting structures that exhibit selectable wearcharacteristics.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an abrasive-impregnated cuttingstructure for use in drilling a subterranean formation. Morespecifically, the abrasive-impregnated cutting structure may comprise aplurality of abrasive particles dispersed within a substantiallycontinuous matrix, wherein the abrasive-impregnated cutting structureexhibits an anisotropic wear resistance. One or more of the amount,average size, composition, strength, properties, shape, quality, andconcentration of the abrasive particles may vary within theabrasive-impregnated cutting structure. Of course, the abrasiveparticles may comprise one or more material or composition, withoutlimitation. In addition, one or more properties of the substantiallycontinuous matrix material may vary within the abrasive-impregnatedcutting structure.

Further, the anisotropic wear resistance may relate to a selecteddirection. For example, anisotropic wear resistance may relate to anexpected direction of engagement of the abrasive-impregnated cuttingstructure with the subterranean formation. In another example,anisotropic wear resistance may relate to a direction substantiallycorresponding to a helix angle associated with the motion of theabrasive-impregnated cutting structure as it is carried by a drag bitduring drilling. Also, the anisotropic wear resistance may be configuredfor forming or retaining a formation-engaging leading edge in responseto cutting engagement with the subterranean formation. Alternatively oradditionally, the anisotropic wear resistance may relate to ananticipated wear direction.

The present invention also relates to a rotary drag bit employing atleast one abrasive-impregnated cutting structure according to thepresent invention. Further, the wear resistance of the at least oneabrasive-impregnated cutting structure may increase substantially inproportion to a radial distance thereof from a longitudinal axis of arotary drag bit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A illustrates a perspective view of a cross-sectionedabrasive-impregnated cutting structure according to the presentinvention;

FIG. 1B illustrates a perspective view of an impregnated segment with across-sectioned end according to the present invention;

FIG. 1C illustrates a perspective view of another embodiment of across-sectioned abrasive-impregnated cutting structure according to thepresent invention;

FIG. 2A illustrates a perspective view of a worn, cross-sectionedabrasive-impregnated cutting structure according to the presentinvention;

FIG. 2B illustrates a perspective view of a worn, cross-sectionedabrasive-impregnated cutting structure according to the presentinvention;

FIG. 2C illustrates a perspective view of a worn, impregnated segmentwith a cross-sectioned end according to the present invention;

FIG. 3A illustrates a perspective view of a cross-sectionedabrasive-impregnated cutting structure according to the presentinvention;

FIG. 3B illustrates a perspective view of an impregnated segment with across-sectioned end according to the present invention;

FIG. 4 illustrates a perspective view of an impregnated segment with across-sectioned end according to the present invention;

FIG. 5A shows a side perspective view of a rotary drag bit according tothe present invention;

FIG. 5B shows a top elevation view of the rotary drag bit shown in FIG.5A;

FIG. 5C shows a schematic, partial side view of a rotary drag bitaccording to one embodiment;

FIG. 6A shows a side perspective view of another rotary drag bitaccording to the present invention;

FIG. 6B shows a top elevation view of the rotary drag bit shown in FIG.6A;

FIG. 7A shows a side perspective view of a further rotary drag bitaccording to the present invention; and

FIG. 7B shows a top elevation view of the rotary drag bit shown in FIG.7A.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to abrasive-impregnated cutting structuresfor use in rotary drag bits for drilling subterranean formations.

Referring now to FIG. 1A of the drawings, a first embodiment of anabrasive-impregnated cutting structure 10, shown in the form of across-sectioned generally cylindrical cutting element 6 disposed on asupport element 16, of the present invention is depicted in aperspective side view, the abrasive-impregnated cutting structure 10including abrasive particles 12 dispersed within substantiallycontinuous matrix 14. It should be appreciated that a cutting element 6may be disposed upon a support element 16 or may be formed integrallywith a rotary drag bit or may be attached to a rotary drag bit by way ofany structure or configuration known in the art, without limitation.

Direction v illustrates an expected direction of engagement of generallycylindrical cutting element 6 with a subterranean formation. Surface 18may generally represent an engagement surface (either initial or worn)for engaging against a subterranean formation. As described in moredetail hereinbelow, direction v may be a function of the drillingconditions. Further, it should be understood that as the generallycylindrical cutting element 6 wears away during use (i.e., drilling asubterranean formation), direction v may change therewith.

According to the present invention, generally cylindrical cuttingelement 6 may be configured to exhibit anisotropic wear resistance withrespect to direction v. “Anisotropic wear resistance,” as used herein,means a wear resistance that varies substantially continuously within abody with respect to at least one direction (in reference to a fixedreference system). For instance, referring to FIG. 1A, moving from apoint lying upon side extent 20 in direction v, a wear resistance of thebody of generally cylindrical cutting element 6, if measured in relationto individual relatively small regions of abrasive particles 12dispersed within substantially continuous matrix 14, may vary therein,with respect to direction v. In contrast, for comparison only, alongside extent 20 (substantially perpendicular to direction v) of generallycylindrical cutting element 6, the wear resistance thereof may besubstantially constant. More particularly, a wear resistance ofabrasive-impregnated cutting structure 10 may decrease with respect todirection v.

Accordingly, abrasive particles 12 within substantially continuousmatrix 14 may vary in at least one of concentration, properties,orientation, composition, strength (e.g., a tensile stress at failure ora compressive stress at failure), shape, and size in relation todirection v. Thus, the wear resistance of generally cylindrical cuttingelement 6 may change in relation to a direction of a fixed referencesystem. More particularly, the wear resistance may be increasing ordecreasing at any given point (in relation to an adjacent point) withingenerally cylindrical cutting element 6 with respect to a selecteddirection of a fixed reference system.

Generally, abrasive particles 12 may comprise synthetic diamond grit,natural diamond grit, cubic boron nitride, silicon nitride, cubic boronnitride, silicon carbide, tungsten carbide, or mixtures thereofExemplary synthetic diamond grit may be commercially available fromElement Six of Shannon, Ireland. Exemplary tungsten carbide material maycomprise relatively fine-grain tungsten carbide powder, such as, forexample, tungsten carbide powder designated by “DM2001,” which iscommercially available from Kennametal Inc., of Latrobe, Pa.Substantially continuous matrix 14 may preferably comprise one or moreof copper, a copper-based alloy, nickel, a nickel-based alloy, cobalt, acobalt-based alloy, iron, an iron-based alloy, silver, or a silver-basedalloy.

Additionally, the abrasive particles 12 may include a coating disposedthereon. Such a configuration may facilitate retention of the abrasiveparticles 12 disposed within substantially continuous matrix 14. Forinstance, such a configuration may increase a so-called “pull-outstrength” of the abrasive particles disposed in substantially continuousmatrix 14. For instance, a coating disposed on the abrasive particles 12may comprise at least one metal selected from Groups IVA, VA, VIA of theperiodic chart or alloys thereof For example, abrasive particles 12 maybe coated with tungsten, nickel, copper, titanium, or combinationsthereof (e.g., in layers or as otherwise known in the art).Alternatively, a coating may comprise a coating as known in the art. Forexample, U.S. Pat. Nos. 5,024,680, 5,224,969, and 5,011,514, thedisclosure of each of which is incorporated by reference herein,disclose examples of conventional coatings that may be applied toabrasive particles 12. Such a coating or coatings may improve adhesionof the substantially continuous matrix to the abrasive particle, mayimprove retention of the abrasive particle within the substantiallycontinuous matrix, or both.

A second embodiment of an abrasive-impregnated cutting structure 50 ofthe present invention is shown in the form of segment 56, in aperspective side view, showing a cross-sectioned end thereof in FIG. 1B,wherein segment 56 includes abrasive particles 52 dispersed withinsubstantially continuous matrix 54. As explained above, direction villustrates an expected direction of engagement of abrasive-impregnatedcutting structure 50 with a subterranean formation. Put another way,direction v shows a direction in which the subterranean formation maypass along upper surface 58 of segment 56. Accordingly, abrasiveparticles 52 may vary in concentration, orientation, strength,composition, properties, shape, or size in relation to the distance fromside extent 60 of segment 56 with respect to direction v. For example,abrasive particles 52 may substantially continuously vary inconcentration, composition, properties, shape, strength, or size inrelation to the distance from side extent 60 of segment 56 with respectto direction v. Thus, abrasive-impregnated cutting structure 50 mayexhibit anisotropic wear resistance, which substantially continuouslyvaries in relation to a distance from side extent 60 along direction v.

Thus, abrasive-impregnated cutting structures 10 and 50 (FIGS. 1A and1B) of the present invention may exhibit anisotropic wear resistance inrelation to a fixed reference system, such as a reference surface, areference plane, or a reference point. In one embodiment, the abrasiveparticles 12 and 52 may be non-uniformly distributed withinsubstantially continuous matrix 14. Generally, a concentration ofabrasive particles 12 and 52 is shown as decreasing with respect todirection v from side extents 20 and 60 toward side extents 22 and 62,respectively. However, although FIGS. 1A and 1B depict a decreasingconcentration of abrasive particles 12 and 52 along direction v, fromside extent 20 and 60, respectively, the present invention is not solimited. Rather, a concentration of abrasive particles 12 and 52 withinsubstantially continuous matrix 14 may decrease or increase at any givenpoint (in relation to an adjacent point) along direction v from sideextent 20 and 60, respectively.

The present invention also contemplates other configurations of anabrasive-impregnated cutting structure that exhibit wear resistanceanisotropy besides those having varying abrasive particle size,concentration, strength, orientation, or combinations thereof within anabrasive-impregnated cutting structure.

For instance, FIG. 1C shows a perspective view of another embodiment ofa cross-sectioned abrasive-impregnated cutting structure 110 includingupper surface 118 and side extents 120 and 122 according to the presentinvention. Abrasive-impregnated cutting structure 110, shown as agenerally cylindrical cutting element 106 disposed on support element116 in FIG. 1C includes abrasive particles 112, which may besubstantially uniformly dispersed throughout substantially continuousmatrix 114, and may be of a substantially similar or equal size, but yetmay be structured to produce anisotropic wear resistance of cuttingelement 106. However, at least one inherent quality related to wearresistance of abrasive particles 112 may vary or change in relation todirection v. Thus, substantially uniformly dispersed abrasive particles112 may be configured to cause the generally cylindrical cutting element106 to exhibit anisotropic wear resistance.

As mentioned above, abrasive particles 12, 52, 112 (FIGS. 1A, 1B, and1C) may preferably comprise a hard, abrasive and abrasion-resistantmaterial, such as, for instance, a superabrasive material. Abrasiveparticles 12, 52, or 112 may each comprise natural diamond, syntheticdiamond, cubic boron nitride, tungsten carbide, or combinations thereof.Thus, an abrasive-impregnated cutting structure according to the presentinvention may include more than one type of abrasive particle, as wellas one or more sizes or quality grades of abrasive particles.

As described above in relation to conventional cutting structures, anabrasive-impregnated cutting structure of the present invention may becast integrally with the body of a drag bit, as by low-pressureinfiltration within a mold, as known in the art. Alternatively, anabrasive-impregnated cutting structure of the present invention may bepreformed or fabricated separately and then brazed or welded to the bodyof a drag bit. Powders comprising abrasive material, matrix material, orboth, may be mixed and preferentially segregated or preferentiallydistributed by the force of earthly gravity in combination withdifferences in density, and, optionally facilitated by vibration. Undersuch a force of segregation, or another force for segregating powders,as known in the art, abrasive particles may be distributed in a desiredconfiguration. Alternatively or additionally, a magnetic field may beused for preferentially segregating or distributing the abrasiveparticles, wherein the abrasive particles comprise a magnetic material.Once a desired distribution of abrasive particles is achieved, animpregnated cutting structure may be pre-pressed or prefabricated (e.g.,infiltrated, exposed to high-pressure isostatic pressure, or otherwiseinfiltrated) for incorporation within a rotary drag bit in the future.

In a further alternative for segregating abrasive particles, so-calledhand packing, as known in the art, or layering may effectively form agradient of abrasive particles. Similarly, prefabricated sheets orlayers of abrasive impregnated materials (e.g., abrasive particleswithin a volatile binder) may be positioned adjacent one another toeffectively form a substantially continuous gradient of at least oneabrasive property in the direction of layering. Alternatively,particles, such as abrasive particles, particles to form a substantiallycontinuous matrix, or both may be sprayed (along with a removable bindermaterial, such as wax or a polymer) into a mold employed for forming arotary drag bit. Optionally, robotic control of the spraying or ofplacement of prefabricated impregnated cutting structures may beemployed for positioning thereof within a mold for fabricating a rotarydrag bit.

In yet a further alternative, a so-called “Selective Laser Sintering”process may be employed for forming an impregnated cutting structure.The selective laser sintering process creates solid three-dimensionalobjects, layer-by-layer, from plastic, metal, diamond, cermets, orceramic powders that are “sintered” or fused using CO2 laser energy. Theinherent materials versatility of SLS technology allows a broad range ofadvanced rapid prototyping and manufacturing applications to beaddressed. The powders may be subsequently infiltrated with anothercompatible material, such as a molten metal, polymer, or other suitableinfiltrant.

Also as mentioned above, the material of the substantially continuousmatrix may be chosen for wettability, mechanical properties, such asabrasion resistance, or both, and may comprise one or more of copper, acopper-based alloy, nickel, a nickel-based alloy, cobalt, a cobalt-basedalloy, iron, an iron-based alloy, silver, or a silver-based alloy. Itshould be understood that the substantially continuous matrix maycomprise different constituents or compositions. For instance, a firstsubstantially continuous matrix material may be melted in and around afirst portion of abrasive particles, while a second substantiallycontinuous matrix material may be melted into a second portion ofabrasive particles, where the first and second section of abrasiveparticles abut one another. Thus, the first material may join directlyto the second material, alloy therewith, or otherwise becomemechanically continuous therewith. Such a configuration may provide asubstantially continuous matrix according to the present invention.

Generally, the present invention encompasses an abrasive-impregnatedcutting structure having an anisotropic wear resistance with respect toa selected direction. In one embodiment of the present invention, a wearresistance of an abrasive-impregnated cutting structure may bestructured for anisotropy with respect to a selected directionsubstantially corresponding to a helix angle associated with the motionof the abrasive-impregnated cutting structure as it is carried by a dragbit during drilling. As known in the art, cutting structures positionedupon a rotary drag bit travel along helical paths as the bit rotates andmoves longitudinally, drilling ahead into the formation. A helix anglemay be determined by a radial position of a cutting structure on thebit, a rate-of-penetration (“ROP”), and a rotational speed of thecutting structure during drilling. Therefore, a helix angle associatedwith an abrasive-impregnated cutting structure may be determinedaccording to its position and the expected rate of penetration androtational speed of the drag bit during use. According to the presentinvention, an abrasive-impregnated cutting structure may be configuredfor exhibiting anisotropic wear resistance with respect to an associatedhelix angle. More particularly, a cutting structure of the presentinvention may exhibit anisotropic wear resistance with respect to apredicted or anticipated helix angle associated therewith.

In one embodiment of the present invention, turning to FIGS. 2A-2C, FIG.2A illustrates a perspective side view of a cross-sectioned, worn,abrasive-impregnated cutting structure 310 including generallycylindrical cutting element 306 disposed upon support element 316,wherein generally cylindrical cutting element 306 comprises abrasiveparticles 312 dispersed within substantially continuous matrix 314.Cylindrical cutting element 306 may be configured according to any ofthe above embodiments of abrasive-impregnated cutting structure 10, 50and 110 described above.

Impregnated cutting structure 310 is shown as having been worn by asubterranean formation by relative movement of upper surface 318 ofgenerally cylindrical cutting element 306 in direction v, oriented at ahelix angle α₁. Helix angle α₁ is shown in relation to reference plane311, which may be oriented substantially perpendicular to a longitudinaldirection of drilling (i.e., the longitudinal axis or rotational axis ofthe drag bit). Thus, FIG. 2A shows that generally cylindrical cuttingelement 306 may be worn from an initial substantially cylindricalgeometry (FIGS. 1A, 1C) to a partially generally cylindrical shapeforming, in part, upper surface 318, which may have a topography whichis substantially parallel to helix angle α₁.

Further, abrasive-impregnated cutting structure 310 may exhibitanisotropic wear resistance with respect to direction v. For instance, awear resistance of abrasive-impregnated cutting structure may decreasein relation to a distance from side extent 320 toward side extent 322along direction v.

In further detail, structuring a wear resistance of abrasive-impregnatedcutting structure 310 to decrease in relation to a distance from sideextent 320, along direction v, may be desirable for producing cutting“clearance.” As shown in FIG. 2B, engagement betweenabrasive-impregnated cutting structure 310 and a subterranean formation(not shown) may form upper boundary 325, which varies from theintersection of direction v with cutting structure 310. Because the wearresistance decreases along direction v, drilling contact with asubterranean formation may cause greater wearing toward rotationallyfollowing edge 331, in relation to leading edge 330. The differencebetween direction v and upper boundary 325 may be termed the“clearance.” Such a configuration may improve the efficiency of drillingwith the cutting structure by preferentially generating or retaining aformation-engaging leading edge 330. Thus, the amount of wear exhibitedby abrasive-impregnated cutting structure 310 may vary in relation to adistance from side extent 320.

In this way, anisotropic wear resistance may be tailored to form a“self-sharpening” geometry. “Self-sharpening,” as used herein, refers toa configuration wherein a leading edge of a cutting structure, inrelation to a direction of movement against a subterranean formation, ispreferentially formed or retained. As may be seen in FIG. 2A, theleading edge proximate to the intersection of side extent 320 and uppersurface 318 may be preferentially retained, by exhibiting greater wearresistance than the wear resistance of other portions of generallycylindrical cutting element 306.

It should be recognized that the clearance depicted in FIG. 2B may implya linearly varying wear resistance. However, the present invention isnot so limited. Rather, the wear resistance of an abrasive-impregnatedcutting structure of the present invention may be structured forproducing a desired clearance having desired characteristics. It shouldalso be noted that although the embodiments shown in FIGS. 2A-2Cgenerally refer to a wear resistance that decreases along direction v,the present invention generally encompasses an abrasive-impregnatedcutting structure having a wear resistance that varies with respect todirection v; thus, an abrasive-impregnated cutting structure may includeregions wherein each region exhibits a wear resistance that isincreasing or decreasing.

Further, FIG. 2C shows segment 356 including abrasive particles 352 andsubstantially continuous matrix 354, wherein segment 356, particularlyabrasive particles 352 dispersed within substantially continuous matrix354, may be configured according to any of the above embodiments ofabrasive-impregnated cutting structures 10, 50, and 110 described above.Segment 356 is shown as having been worn by relative movement of wornupper surface 358 of segment 356 in direction v, oriented at a helixangle α₂, wherein helix angle α₂ is shown in relation to reference plane351, which is oriented substantially perpendicular to a longitudinaldirection of drilling (i.e., the longitudinal axis or rotational axis ofthe drag bit). More specifically, segment 356 may have been worn from aninitial substantially rectangular geometry (FIG. 1B) to a partiallygenerally rectangular or trapezoidal shape forming, in part, worn uppersurface 358, which is oriented substantially at helix angle α₂.

Further, abrasive-impregnated cutting structure 350 may exhibitanisotropic wear resistance with respect to direction v. For instance, awear resistance of abrasive-impregnated cutting structure 350 maydecrease with respect to direction v from side extent 360 toward sideextent 362. Configuring the wear resistance of abrasive-impregnatedcutting structure 350 to decrease in relation to a distance from sideextent 360, along direction v, may be desirable for producing cutting“clearance,” as described above in relation to FIG. 2B. Thus, the amountof wear exhibited by segment 356 in response to cutting interaction witha subterranean formation may be proportional to a distance from sideextent 360 along direction v.

In a further aspect of the present invention, the wear resistance of anabrasive-impregnated cutting structure may vary with respect to ananticipated direction of wear. Particularly, FIGS. 3A and 3B showabrasive-impregnated cutting structures 310 and 350 generally as shownin FIGS. 2A and 2C, respectively, however, the wear resistance thereofmay vary with respect to an anticipated wear direction, w. In furtherdetail, anticipated wear direction w may be the direction of thechronological change in shape (i.e., shrinkage) of abrasive-impregnatedcutting structure 310 in response to cutting or drilling contact with asubterranean formation.

For instance, if abrasive-impregnated cutting structure 310 wears toform an upper surface 318 that is substantially parallel to direction v,an anticipated wear direction w may be substantially perpendicularthereto. As shown in FIG. 3A, generally cylindrical cutting element 306may be configured with a wear resistance which increases from upperboundary 325 with respect to (i.e., in the direction of) anticipatedwear direction w toward lower boundary 326. Similarly, as shown in FIG.3B, segment 356 may be configured with a wear resistance that increasesfrom upper edge 359 with respect to (i.e., in the direction of)anticipated wear direction w.

In a further aspect of the present invention, a chronologicalprogression of wear of an abrasive-impregnated cutting structure inresponse to drilling contact with an expected subterranean formation maybe predicted by simulation, modeling, prediction, or empiricalobservation. The chronological progression of cutting structure wear mayindicate an anticipated wear surface with respect to the cuttingstructure. Accordingly, the predicted direction of wear may be used forstructuring of an anisotropic wear characteristic of anabrasive-impregnated cutting structure for use on a rotary drag bit.

Further, the wear resistance anisotropy of an abrasive-impregnatedcutting structure may be selected with respect to the subterraneanformation that the drag bit upon which it is carried is intended fordrilling through. For instance, such a configuration may cause theabrasive-impregnated cutting structure to become, via wearing, more orless aggressive. Further, combination of varying wear resistance as wellas varying geometry (e.g., tapered or otherwise changing geometry) byway of wearing of an abrasive-impregnated cutting structure may resultin an abrasive-impregnated cutting structure that becomes, via wearing,more suitable for drilling through different subterranean formationssubstantially as they may be encountered.

In one example, a relatively non-aggressive but relatively wearresistant abrasive-impregnated cutting structure may drill through arelatively hard subterranean formation, causing wearing of theabrasive-impregnated cutting structure. In turn, such wearing may causethe abrasive-impregnated cutting structure to become more aggressive, byway of, for instance, reducing the contact area between theabrasive-impregnated cutting structure and the subterranean formation.The change in aggressiveness of the abrasive-impregnated cuttingstructure due to wear thereof may be structured to substantiallycorrespond to a change in the subterranean formation. For example,predictive wear models, simulations, or empirical data may be used tosize, position, structure, and select the materials and characteristicsof an abrasive-impregnated cutting structure, so that changes in atleast one characteristic thereof due to wearing substantially correspondto a change in the subterranean formation (e.g., drilling through shaleinto sandstone).

As a further extension of the present invention, an abrasive-impregnatedcutting structure according to the present invention may exhibit wearresistance anisotropy with respect to both an anticipated direction ofengagement with a subterranean formation (i.e., helix angle associatedwith an anticipated motion of the abrasive-impregnated cuttingstructure) and an anticipated direction of wear. Specifically, FIG. 4shows abrasive-impregnated cutting structure 280, or, more specifically,segment 281, illustrated in a perspective side view, showing across-sectioned end thereof wherein the wear resistance of segment 281varies in relation to direction d from edge 293. Segment 281 includesside extent 284, side extent 286, upper surface 291, and lower surface287. Segment 281 also comprises abrasive particles 282 dispersed withinsubstantially continuous matrix 288. Abrasive particles 282 are depictedas varying in concentration, composition, strength, characteristics,shape, or size in relation to a distance from edge 293 along directiond. Direction d may be a vector combination of an anticipated weardirection (w, as shown in FIGS. 3A and 3B) and direction v (orientedsubstantially at an associated helix angle, as shown in FIGS. 2A-2C).Accordingly, in one embodiment, the wear resistance of segment 281 maybe at a maximum at edge 293, and decrease in relation to a distancetherefrom along direction d.

Also, it should be noted that a method of manufacturing of anabrasive-impregnated cutting structure of the present invention isencompassed by the present invention. For instance, a mold may be filledwith abrasive particles and, optionally, matrix particles and orientedso that a primary segregation direction, a direction with respect towhich the abrasive particles may be segregated (e.g., the direction ofthe force of gravity), is generally aligned with a direction of desiredwear resistance anisotropy, such as v, w, or d, as discussed above. Theabrasive particles may be held in place by a wax or other binder andsubsequently removed from the oriented mold and placed in a drag bitmold for infiltration therewith. Alternatively, the abrasive particlesmay be infiltrated in the oriented mold and brazed, welded, or otherwiseaffixed to a rotary drag bit.

In summary, as may be seen from the foregoing examples, manyconfigurations are encompassed by the present invention for varying thewear resistance of an abrasive-impregnated cutting structure.Particularly, an abrasive-impregnated cutting structure according to thepresent invention may be structured for exhibiting an anisotropic wearresistance with respect to a helix angle, an anticipated wear direction,or a combination thereof. Further, such anisotropic wear resistance maybe configured or structured in relation to drilling interaction with anexpected subterranean formation or, more specifically, cuttinginteraction with differing regions thereof.

As may be appreciated by one of ordinary skill in the art, wearresistance of an impregnated cutting structure may be dependent on theconditions under which the wear occurs. Modeling and conceptual theorieshave been developed regarding wear mechanisms of impregnated cuttingstructures. For example, a so-called “three-body” abrasion modelconsiders the properties of the substantially continuous matrix, anabrasive particle at least partially suspended within the continuousmatrix, and the cuttings of the material to be abraded (i.e., thesubterranean formation). Of course, such a model or concept is typicallyan approximation when compared to the complexity of the actualconditions surrounding wear of an impregnated cutting structure. Someadditional factors that may affect the wear behavior of an impregnatedcutting structure may include: a distance that an abrasive particleprotrudes from the substantially continuous matrix (otherwise termed“exposure”), the speed at which the abrasive particle encounters thesubterranean formation, the forces between the abrasive particle and thesubterranean formation, and the effectiveness of fluid for cooling theabrasive particles and carrying cuttings of the subterranean formationaway from the impregnated cutting structure.

Thus, the direction of engagement and nature of contact between twobodies or materials may influence or determine wear resistancetherebetween. Of course, the properties of the materials may alsoinfluence or determine wear resistance between two contacting bodies.Therefore, while examples of preferential retention of aformation-engaging leading edge in response to wearing of anabrasive-impregnated cutting structure may be given herein, none of suchexamples are limiting.

At least one abrasive-impregnated cutting structure of the presentinvention may be positioned and affixed to a rotary drag bit. Forinstance, as shown in FIGS. 5A and 5B, showing a perspective side viewand a top elevation view of a rotary drag bit 410 of the presentinvention, respectively. Referring to FIG. 5A, rotary drag bit 410includes a body 420 and shank 426 for connection to a drill string (notshown), the shank 426 extending therefrom opposite to bit face 419. Eachof the plurality of blades 412 extend generally radially outwardly withrespect to longitudinal axis 408 to a gage region 425. Also, both fluidcourses 416 extend to junk slots 418 formed between circumferentiallyadjacent blades 412. Gage regions 425 may include natural diamonds 414,but may also include, additionally or alternatively, at least one ofsynthetic diamond and cemented tungsten carbide, wherein the cementedtungsten carbide may be in the form of “bricks,” as known in the art.

During operation, rotary drag bit 410 may be affixed to a drill string(not shown), rotated about longitudinal axis 408 in direction 409 (FIG.5B), and may translate along the direction of longitudinal axis 408 intoa subterranean formation (not shown), as known in the art. Also,drilling fluid may be communicated through one or more apertures 422from the interior of rotary drag bit 410 to the face 419 (FIG. 5A)thereof, moving along fluid courses 416, into junk slots 418, andultimately upwardly within the annulus formed between the drill stringand the borehole that is formed by the rotary drag bit 410 duringdrilling. In this way, cuttings of the formation may be transportedwithin the drilling fluid and the cutting structures on the rotary dragbit 410 may be cooled.

At least a portion of at least one of the plurality of blades 412 of therotary drag bit 410 may carry or may comprise an abrasive-impregnatedcutting structure that exhibits anisotropic wear resistance.Particularly, at least a portion of at least one of the blades 412comprising an abrasive-impregnated cutting structure may be structuredor configured to exhibit an anisotropic wear resistance. Particularly,the blade 412A of blades 412 with labeled leading edge 411 and trailingedge 413 is shown as comprising an abrasive-impregnated cuttingstructure.

In one embodiment, the amount of wear exhibited by blade 412A may varyfrom less wear to greater wear in relation to the distance from theleading edge 411 to the trailing edge 413 of a blade 412. Thus, theanisotropic wear resistance may be configured to form a“self-sharpening” geometry, wherein the leading edge 411 of blade 412Ais preferentially formed or retained. Optionally, anisotropic wearresistance of blade 412A may be configured to exhibit anisotropy withrespect to an anticipated direction of wear, as discussed above.

Although the anisotropic wear resistance may be configured to vary inrelation to the path that is traversed by the formation at a positionalong the leading edge 411 to the trailing edge 413 of blade 412A, whichwould be generally circular and opposite to the direction of rotation409, such correspondence to the direction of cutting is not required.Rather, the wear resistance may vary (exhibit anisotropy) along a curvedor straight path from the leading edge 411 to the trailing edge 413 ofblade 412A, without limitation. As may be appreciated, there are manydifferent configurations that may cause at least a portion of blade 412Ato form a “self-sharpening” geometry, wherein a leading edge 411 isstructured or configured to be preferentially formed or retained inresponse to wearing via contact with a subterranean formation. Further,optionally, the wear resistance of blade 412A may be configured forgenerating a selected clearance. Additionally or alternatively, the wearresistance of blade 412A may be configured to vary with respect to ananticipated direction of wear.

In another aspect of the present invention, the wear resistance of anabrasive-impregnated cutting structure carried by rotary drag bit 410may be configured according to a region of the rotary drag bit 410. Forexample, as shown in FIG. 5C, which depicts a schematic, partial sideview of rotary drag bit 410, a wear resistance of anabrasive-impregnated cutting structure may be selectively tailored toincrease or decrease within a selected region 449 of rotary drag bit 410with respect to a selected direction. In one embodiment, a wearresistance of an abrasive-impregnated cutting structure may beselectively tailored to increase or decrease within a selected region449 of rotary drag bit 410 with respect to a radial direction R (i.e.,radially away from longitudinal axis 408), as shown in FIG. 5C. Thus, inone embodiment, the wear resistance of an abrasive-impregnated cuttingstructure carried by rotary drag bit 410 may be configured to increasegenerally from an outer diameter to a shoulder region of the rotary dragbit 410. Alternatively, a wear resistance may vary in relation to alongitudinal direction (i.e., substantially parallel to longitudinalaxis 408).

In a further alternative, a wear resistance of an abrasive-impregnatedcutting structure carried by rotary drag bit 410 may be configuredaccording to an amount of formation that is encountered therewith. Asmay be appreciated, the volume of subterranean formation encountered bya cutting structure may be related to, at least in theory, the size ofthe imaginary circles that it rotates about, centered about the axis ofrotation of the rotary drag bit during use, the imaginary circlessuperimposed over the face of the rotary drag bit. Thus, as the radialdistance from the longitudinal axis of the rotary drag bit increases,the size of such circles increases and so does the volume of thesubterranean formation encountered by a cutting structure so positioned.Accordingly, the present invention contemplates that the wear resistanceof an abrasive-impregnated cutting structure may be selectively tailoredin relation to its radial position on the bit face.

Thus, a wear resistance of an abrasive-impregnated cutting structureaccording to the present invention may increase substantially inrelation to a radial distance from the longitudinal axis of the rotarydrag bit 410, because a volume of the subterranean formation removed bya given region thereof is substantially proportional to a square of itsradial distance from the longitudinal axis of the rotary drag bit 410.Thus, the concentration, composition, strength, characteristic, shape,or size of abrasive particles may increase as a function of a radialdistance from the longitudinal axis of the rotary drag bit 410.Specifically, the amount, average size, or concentration of abrasiveparticles may increase roughly in proportion to a radial distance fromthe longitudinal axis, which is related to the amount of subterraneanformation that would be removed at a radial position upon the rotarydrag bit 410. Alternatively, the wear resistance of theabrasive-impregnated cutting structure may be increased as a function ofa radial distance from the longitudinal axis of the rotary drag bit 410.Alternatively or additionally, an inherent quality related to wearresistance may increase substantially in proportion to a radial distancefrom the longitudinal axis of rotary drag bit 410.

Additionally, as the radial distance of the cutting structure increasesfrom the axis about which the rotary drag bit 410 is rotating (ideallythe longitudinal axis 408), the volume, depth of cut, and cutting speedin relation to the subterranean formation that is encountered andremoved therewith for a given longitudinal distance of a subterraneanformation drilled by the bit varies. Therefore, it may be advantageousto configure an abrasive-impregnated cutting structure positioned at aradial position or region so that its relative wear (distance ofmaterial removed) may be generally equal to or balanced in relation tothe relative wear of another abrasive-impregnated cutting structurepositioned at a different radial position. Such a configuration, ifemployed upon the entire abrasive-impregnated cutting structure of arotary drag bit 410 may be advantageous in reducing the occurrence ofso-called “ring-out,” which is a term used to describe the condition ofa portion of the abrasive-impregnated cutting structure wearing or beingdamaged to a point to which cutting no longer occurs and occursprematurely to the remaining cutting structure wearing to about the sameamount. Such a configuration may also maintain a substantially congruousprofile shape in relation to the initial profile shape as the rotarydrag bit wears. Accordingly, in one specific example, a concentration,composition, characteristic, shape, a size, a strength, or an inherentquality related to wear resistance of abrasive particles of animpregnated cutting structure may increase as a function of a square ofa radial distance from the longitudinal axis of the rotary drag bit 410.Such a configuration may compensate for the variation of a volume of thesubterranean formation removed by a particular region (i.e., a radialposition) of the rotary drag bit 410.

In another aspect of the present invention, it may be advantageous toconfigure an abrasive-impregnated cutting structure to exhibit varyingwear resistance in relation to the properties of the formation beingencountered. For instance, a first formation may be encountered that iseffectively drilled by a first abrasive material within a firstsubstantially continuous matrix, while a second formation may beencountered subsequently that is effectively drilled by a secondabrasive material within a second substantially continuous matrix.Therefore, a first impregnated material comprising a first abrasivematerial and a first substantially continuous matrix may be sized andconfigured to drill a first section of subterranean formation, while thesecond impregnated material comprising a second abrasive material withina second substantially continuous matrix may be sized and configured todrill a subsequent, second section of subterranean formation. Of course,additional abrasive-impregnated material configurations corresponding toadditional subsequent subterranean formations may be included.

In yet another aspect of the present invention, the abrasive-impregnatedcutting structure of a rotary drag bit may be configured with respect toa cutting speed at the radial position on the rotary drag bit.Specifically, within region 440, at radial positions proximate thelongitudinal axis 408 of the rotary drag bit 410, the cutting speed maybe relatively low, which may allow for relatively slow, sliding contactbetween the formation and the substantially continuous matrix of anabrasive-impregnated cutting structure, since the speed at which aportion of the rotary drag bit 410 contacts the subterranean formationis equal to, ideally, the radial position of the portion multiplied bythe rotational velocity of the rotary drag bit. Therefore, near thelongitudinal axis 408 of the rotary drag bit 410 (region 440), themagnitude of cutting speed may be relatively low. Also, local forces oneach of the abrasive particles may be higher proximate the longitudinalaxis, due to the depth of cut (related to the helix angle) and the speedat which the abrasive particle travels during drilling.

Of course, many other factors may affect a wearing behavior of a portionof an impregnated cutting structure of a rotary drag bit. For example,the number of blades, hydraulic environment, type of formation, androtary drag bit motion during drilling may affect the wear behavior of aportion of an impregnated cutting structure of a rotary drag bit.Accordingly, configuring an anisotropic wear property of an impregnatedcutting structure may be an iterative process that is developed andrefined through repeated study of used or “dull” rotary drag bits, orother analysis mechanisms, as known in the art.

According to the present invention, by way of example, it may beadvantageous to select an increased particle size of the abrasivematerial within region 440, so the individual particles thereof may beexposed or protrude from the substantially continuous matrix, instead ofwearing substantially evenly therewith. Put another way, larger abrasivematerial particles within a substantially continuous matrix may beemployed in regions of an abrasive-impregnated cutting structurepositioned near the longitudinal axis of the rotary drag bit 410, whilefiner or smaller abrasive material particles may be employed in regionsof an abrasive-impregnated cutting structure positioned radiallyoutwardly from region 440. Such a configuration may prevent undesirablefriction from relatively slow sliding contact between the formation anda substantially smooth surface of an abrasive-impregnated cuttingstructure within the region 440 of the rotary drag bit 410 proximate tothe longitudinal axis 408 by allowing the protruding abrasive materialparticles to provide exposure or stand-off between the formation and thesubstantially continuous matrix of the abrasive-impregnated cuttingstructure. Of course, the present invention is not limited by theabove-described example and an increased particle size of the abrasivematerial may be exhibited in any selected region (e.g., cone, nose,shoulder, gage, etc.) of a rotary drag bit.

Of course, any of the embodiments of abrasive-impregnated cuttingstructures described in relation to FIGS. 1A-4 may be employed on arotary drag bit 410 of the present invention, without limitation. Asmentioned above, an abrasive-impregnated cutting structure of thepresent invention may comprise a discrete, abrasive-impregnated cuttingstructure, an impregnated segment, or another abrasive-impregnatedcutting structure as known in the art.

Referring now to FIGS. 6A and 6B, another embodiment of a rotary dragbit 510 of the present invention is shown in a perspective side view anda top elevation view, respectively. Rotary drag bit 510 includes a body520 and shank 526 for connection to a drill string (not shown). Rotarydrag bit 510 further includes a plurality of blades 512 extendinggenerally radially outwardly with respect to longitudinal axis 508 tocorresponding gage regions 525. Fluid courses 516 extend to junk slots518 formed between circumferentially adjacent blades 512.

During operation, rotary drag bit 510 may be affixed to a drill string(not shown), rotated about longitudinal axis 508 in direction 509 (FIG.6B), and may translate along the direction of longitudinal axis 508 intoa subterranean formation (not shown), as known in the art. Also,drilling fluid may be communicated through central aperture 522 from theinterior of rotary drag bit 510 to the face 519 (FIG. 6A) thereof, thedrilling fluid moving along fluid courses 516, into junk slots 518, andultimately upwardly within the annulus formed between the drill stringand a borehole formed by the rotary drag bit 510 during drilling.Alternatively, rotary drag bit 510 may include more than one aperturefor communicating drilling fluid from the interior thereof to theplurality of blades 512 and face 519 thereof In this way, cuttings ofthe formation may be transported within the drilling fluid and thecutting structures on the rotary drag bit 510 may be cooled.

Further, at least a portion of at least one of the plurality of blades512 may comprise an abrasive-impregnated cutting structure of thepresent invention. As explained above, the abrasive-impregnated cuttingstructure may comprise many different configurations and may beconfigured in relation to the subterranean formation to be drilled. Inaddition, a rotary drag bit according to the present invention is notlimited to abrasive-impregnated cutting structures, but may also includepolycrystalline diamond cutting elements or thermally stable diamondelements. Such a configuration may provide enhanced operationalcharacteristics during drilling of a subterranean borehole.

Any of the embodiments of abrasive-impregnated cutting structuresdescribed in relation to FIGS. 1A-4 may be employed on a rotary drag bit510 of the present invention, without limitation. As mentioned above, anabrasive-impregnated cutting structure of the present invention maycomprise a discrete, abrasive-impregnated cutting structure, anabrasive-impregnated segment, or another abrasive-impregnated cuttingstructure as known in the art.

In a further embodiment, an exemplary rotary drag bit 610 of the presentinvention, as shown in FIGS. 7A and 7B in a perspective side view and atop elevation view, respectively, may include discrete,abrasive-impregnated cutting structures 624, which are formed ofimpregnated material. Discrete, abrasive-impregnated cutting structures624 may comprise rotary drag bit 610; accordingly, discreteabrasive-impregnated cutting structures 624 may be formed as a portionof the rotary drag bit 610, as by infiltration therewith, or,alternatively, may be infiltrated, hot pressed, or otherwise fabricatedseparately and then affixed to the rotary drag bit 610 by brazing orpress-fitting, as discussed above. Discrete, abrasive-impregnatedcutting structures 624 may be generally shaped with circular outer endsand oval shaped bases, but other geometries are also contemplated.Further, the discrete, abrasive-impregnated cutting structures 624 maycomprise a variable or changing cross-section. It is also noted that thespacing between individual discrete, abrasive-impregnated cuttingstructures 624, as well as the magnitude of the taper from the outermostends 626 to the blades 612, may be varied to change the overallaggressiveness of the bit 610 as it wears during operation. It isfurther contemplated that one or more of such discrete,abrasive-impregnated cutting structures 624 may be formed to havesubstantially constant cross-sections if so desired depending on theanticipated application of the rotary drag bit 610.

Rotary drag bit 610 includes a body 620 and may include a shank (notshown) for connection to a drill string (not shown). Face 619 includes aplurality of blades 612, wherein each of the plurality of blades 612extend generally radially outwardly with respect to longitudinal axis608 to a gage region 625. Blades 612, in this embodiment, however, referto generally radially arranged groups of discrete, abrasive-impregnatedcutting structures 624. More generally, discrete, abrasive-impregnatedcutting structures 624 may be arranged in concentric or spiral fashion.Fluid courses 616 extend to junk slots 618 formed betweencircumferentially adjacent blades 612. Accordingly, during operation,rotary drag bit 610 may be affixed to a drill string (not shown),rotated about longitudinal axis 608 in direction 609 (FIG. 7B), and maytranslate along the direction of longitudinal axis 608 into asubterranean formation (not shown), as known in the art.Contemporaneously, drilling fluid may be communicated through centralaperture 622 from the interior of rotary drag bit 610 to the face 619thereof, moving along fluid courses 616, into junk slots 618, andultimately upwardly within the annulus formed between the drill stringand a borehole formed by the rotary drag bit 610 during drilling. Ofcourse, rotary drag bit 610 may include a plurality of apertures 622disposed at different positions on face 619 for communicating drillingfluid from the interior thereof to the blades 612 and face 619 thereoffor cooling the discrete, abrasive-impregnated cutting structures 624,as well as transporting formation cuttings to the surface of theformation.

Generally, at least one of the abrasive-impregnated cutting structures624 may be structured or configured to exhibit anisotropic wearresistance. For example, discrete, abrasive-impregnated cuttingstructure 624A may exhibit anisotropic wear resistance that may beconfigured to vary in relation to the path that is traversed by theformation at a position along the leading edge 611 to the trailing edge613, which may be generally circular and opposite to the direction ofrotation 609. More generally, the wear resistance ofabrasive-impregnated cutting structure 624A may vary or exhibitanisotropy between the leading edge 611 to the trailing edge 613.Accordingly, optionally, discrete, abrasive-impregnated cuttingstructure 624A may be configured to form a “self-sharpening” geometry,wherein the leading edge 611 is preferentially formed or retained inresponse to wearing via contact with a subterranean formation.Additionally or alternatively, the wear resistance, geometry, andposition of discrete, abrasive-impregnated cutting structure 624A may beconfigured in relation to expected changes in the subterraneanformation, as discussed above regarding other embodiments of the presentinvention. Further, the wear resistance of abrasive-impregnated cuttingstructure 624A may vary or exhibit anisotropy with respect to ananticipated direction of wear.

Further, any of the embodiments of abrasive-impregnated cuttingstructures described in relation to FIGS. 1A-4 may be employed on arotary drag bit 610 of the present invention, without limitation. Asdiscussed hereinabove, an abrasive-impregnated cutting structure of thepresent invention may comprise a discrete, abrasive-impregnated cuttingstructure, an abrasive-impregnated segment, or anotherabrasive-impregnated cutting structure as known in the art.

Although the foregoing description contains many specifics, these shouldnot be construed as limiting the scope of the present invention, butmerely as providing illustrations of some exemplary embodiments.Similarly, other embodiments of the invention may be devised that do notdepart from the spirit or scope of the present invention. Features fromdifferent embodiments may be employed in combination. The scope of theinvention is, therefore, indicated and limited only by the appendedclaims and their legal equivalents, rather than by the foregoingdescription. All additions, deletions, and modifications to theinvention, as disclosed herein, which fall within the meaning and scopeof the claims, are to be embraced thereby.

What is claimed is:
 1. An abrasive-impregnated cutting structure for useon a rotary drag bit for drilling a subterranean formation, comprising:a plurality of abrasive particles dispersed within a body comprising asubstantially continuous matrix and at least one of configured andarranged to vary in at least one physical characteristic in proportionto a distance from a first location within the body with respect to atleast one direction therethrough; wherein the abrasive-impregnatedcutting structure exhibits an anisotropic wear resistance.